1. Field of the Invention
The present invention relates to methods for printing three-dimensional objects, for example using inkjet printing. The method is particularly suited for the case where the three-dimensional object is a relief print master such as a flexographic print master or sleeve. The method consists of first calculating a set of image layers and constructing the print master by printing these layers on top of each other.
The invention specifically relates to methods to reduce image quality artifacts such as banding and streaking that are related to individual nozzles of a printhead and that can occur when intermediate layers are printed on top of each other. Prior art examples of such methods are usually referred to as shingling and interlacing methods.
The present invention also relates to a corresponding printing apparatus for creating the three-dimensional object.
2. Description of the Related Art
Three-dimensional printing is a method for creating three-dimensional objects by depositing or forming thin layers of material in succession on top of each other so as to build up the desired three-dimensional structure. It is sometimes called “Rapid Prototyping and Manufacturing” (RP&M).
Various methods have been devised to create the thin layers.
One technique makes use of a bath of polymerisable liquid material. A thin upper layer of the liquid is cross-linked or hardened in some way, e.g. via laser light in a pattern which is the same as a cross-section through the object to be formed. The laser spot is moved across the surface in accordance with a digital representation of the relevant cross section. After one layer is completed, the liquid level is raised over a small distance and the process is repeated. Each polymerised layer should be sufficiently form-stable to support the next layer.
In another technique powder is dusted onto a substrate and the powder is coalesced by some means, e.g. by selectively heating or hardening the powder, in accordance with a cross-section of the object to be formed.
In yet another method, cross-linkable or hardenable material is deposited in the form of drops which are deposited in a pattern according to the relevant cross-section of the object to be formed.
Still another method involves dispensing drops of molten material at an elevated temperature which then solidify on contact with the cooler work piece.
More information on rapid prototyping, three-dimensional printing, additive fabrication, tooling, and rapid manufacturing is also found in the Wohlers Report 2008, edited and published by Wohlers Associates, Inc., OakRidge Business Park 1511 River Oak Drive in Fort Collins, Colo. 80525 USA.
Printing plates (also referred to as print masters or print forms) are traditionally manufactured using a combination of image wise exposure by a laser or light source followed by a chemical or physical developing step. Such plates are used in a variety of printing methods, such as flexographic printing, letterpress, offset or gravure printing.
Flexographic printing or flexography is a printing process where a cylindrical flexible print master transfers a fast drying ink from an anilox roll to the printable substrate. The print master can be a flexible plate mounted on a cylinder or it can be a cylindrical sleeve.
Flexographic printing plates have the features that define the image that is to be printed in relief, which means that the ink printing area is raised relative to the non ink printing area. An advantage of flexographic printing is that almost any material that can run through a web press can be printed on in this way, including hard surfaced material such as corrugated fiberboard, plastic films or even metal sheets.
European patent application with publication number EP 1 428 666 by Verhoest et al. teaches making a flexographic printing plate using an inkjet apparatus. The plate is formed by applying subsequently on a substrate at least two image-wise layers of polymerisable ink by an inkjet printer. Between the application of the first and second layers, the first layer is immobilized by initiating a polymerization of the ink using a UV source.
Because of manufacturing tolerances, the volume, speed and direction of ink droplets ejected by the nozzles of an inkjet printhead may slightly vary between individual nozzles. It is well known in two-dimensional printing that in absence of any compensating measures this may lead to image quality artifacts such as banding and streaking that are correlated with individual nozzles.
In two-dimensional printing, such banding and streaking artifacts are effectively suppressed by means of interlacing and shingling techniques. One such technique is disclosed in for example the U.S. Pat. No. 6,679,583 assigned to Agfa-Graphics NV. According to the teachings of this patent, pixels on a single row or column of the image are printed by different nozzles. As a result, the effects of individual nozzle variation are spatially diffused so that they become less noticeable. This effectively suppresses the visibility of banding and streaking.
The shingling and interlacing techniques that are disclosed in the prior art can also be used to suppress banding and streaking artifacts in three-dimensional printing. In that case a next intermediate layer is printed on top of a previous intermediate layer, whereby each layer is printed using shingling and interlacing.
For optimal printing performance, it is required that flexographic print plates or sleeves have a top layer that is flat and even. Additionally it is required that mechanical qualities such as strength, resilience and elasticity of the print master are isotropic.
It was found that prior art shingling and interlacing techniques do not always result in the desired degree of flatness and evenness, resulting in a mottled appearance of an image that is printed with such a flexographic printing plate.
From this follows that there is a need for a solution to improve evenness and flatness of the top layer of a flexographic printing plate that is created using three-dimensional printing. In general there is a need to improve the evenness of three-dimensional objects that are created by stackwise printing intermediate layers.
An additional problem with a prior art technique for creating a flexographic print master is that the mechanical properties are insufficiently isotropic.
For example, the adhesion between the subsequent intermediate layers that make up a print master may not be optimal.
Hence there is a need for a method that improves the isotropy of the mechanical characteristics of a flexographic print master that is created with an inkjet system.